The present invention relates to a polishing system for polishing objects such as wafers, glass substrates, and the like which have films on their surfaces.
For example, in the manufacturing process of Large Scale Integration (LSI), wiring patterns for forming transistors, capacitors, and the like are formed on a silicon wafer substrate or the like, and further an insulating film is formed over the whole surface. At such time, the underlying wiring pattern causes parts of the surface of the insulating film to be uneven. In the recent LSI manufacturing technique, in order to make the wiring pattern multi-layered, the insulating film having the uneven surface is required to be flattened. In order to flattenier level the insulating film having the uneven parts, a Chemical Mechanical Polishing (CMP) method is generally utilized.
In case of carrying out successively polishing of the films such as an insulating film having the uneven parts formed on the wafer by CMP method, there is used in general a method of previously measuring polishing speed, estimating the polishing time based on the polishing speed, and effecting polishing at the estimated polishing time. According to this method, in case the polishing speed and uniformity in the wafer surface thereof are stabilized with time, the wafer can be polished in stabilized state for a long duration.
However, the polishing speed varies depending on the change with time of the surface condition of a polishing pad, irregularity of quality of the polishing slurry, and the like Accordingly, in case of a necessity to control polishing amount in high precision, the change with time of the polishing speed is required to be considered.
As a method for considering the change with time of the polishing speed, in case of polishing the wafer provided with a film having the uneven parts successively, there is proposed a method of obtaining thickness of the film before and after the polishing with an optical film thickness gauge, obtaining polishing speed, and then reset polishing time (Japanese Patent Application Laid-Open No. 8-17768 (1996).
According to this method, the polishing time can be varied in accordance with the change with time of the polishing speed, and therefore the polishing amount can be kept a constant value.
According to this method, however, the wafer to be the product such as LSI (hereinafter to be referred to as "product wafer"), i.e., wafer provided with a film having convex and concave regions, is used for measuring the polishing speed, so that it involves the following problems.
(1) Because of the fine patterns formed on the product wafer, even on slight displacement of the measuring position, change occurs in the film base, such as for example from silicon to aluminum. For this reason, if there is any displacement in the measurement position, then the measured amount of the film thickness becomes inaccurate.
(2) Depending on convex or concave part, if there is a slight displacement in the measurement position, the film thickness varies to a considerable degree. Therefore, it is difficult to measure the film thickness accurately.
(3) In recent years, there are frequently cases where, with the object of preventing reflection, an intermediate film of TiN or the like is used for the base of the film. In such a case, due to the presence of the intermediate film, precision of film thickness measurement is degraded.
As described above, in case a product wafer is used for measuring the polishing speed, there is a problem to make the film thickness measurement value inaccurate. When the film thickness measurement value is inaccurate, evaluation of the polishing amount (polishing speed) becomes inaccurate, and proper determination of the polishing time cannot be made, so that the desired amount of polishing can not be obtained.
In order to obviate such problem of measurement, there may be conceived to measure the film thickness of only the peripheral part of the wafer on which no LSI device is formed, i.e., without having convex or concave part. However, by only he measurement of the film thickness of the peripheral part of the wafer, it is not possible to evaluate the polishing situation at the central part of the wafer on which the LSI device is practically to be formed. Furthermore, it is also difficult to evaluate the uniformity inside the wafer surface which is an important index.
Furthermore, in case of replacement of the polishing pad in the CMP device, the polishing process for the product wafer is started after the following trial operation.
First, in order to stabilize the wafer polishing situation, break-in polishing is carried out. This is a processing to place a dummy wafer for break-in polishing such as silicon wafer (hereinafter to be referred to as "dummy wafer") on a sample stage and continue polishing to make the polishing slurry sufficiently be absorbed by the polishing pad.
The time of completion of the break-in polishing (i.e., the starting point of polishing the product wafer) is determined by experience by the worker. However, the worker's judgment by experience is apt to cause errors.